Living in some kind of social grouping is not unusual among mammals and is certainly not special to primates. Wolves, African hunting dogs and lions all exhibit complex intra-group relationships whose sophistication may in some aspects rival or even excel that seen in primates. Even vampire bats demonstrate a form of social behaviour and of a type rare among primates - the voluntary sharing of food. A vampire returning replete with blood may regurgitate some of this liquid largesse into the begging mouths of its less fortunate (and probably unrelated) fellows back at the roost. Such apparently selfless generosity to non-kin is rare among any animals and very unusual in primates (although with the vampire bat it may not be as selfless as it initially appears, as today's blood-    giver may be tomorrow's receiver). Even such an unlikely animal as the naked mole-rat exhibits strangely insect-like habits. It has a complex ant-like caste system in which the sole breeding 'queen rat' suppresses breeding in her followers.

Primate social behaviour resembles neither that of the vampire nor of the bizarre mole-rat. In fact in many cases the present lack of field studies means that we are uncertain just how the social systems of many species actually function; but increased human presence in the field is rapidly yielding much more information on this and other topics.

Social behaviour is based on the 'home range', the area which is regularly used by the animal or group of animals for all their regular activities. The home range may not be exclusive, but may be shared with other individuals or groups of the same and other species."Certain parts of the home range may be used more than others - some primates only regularly use the 10 per cent or so containing the richest food resources and favourite sleeping sites. The borders of a home range are not defended, but those of a 'territory' most certainly are. Territorial primates use many methods for guarding against trespass across their boundaries to ensure that they have exclusive use of their reserved living space.

At its simplest, primate social behaviour may appear at first sight to be non-social, for many of the nocturnal prosimians appear to lead noticeably solitary lives. However, just because a weasel lemur or bushbaby is almost always encountered feeding alone at night does not mean that it lacks regular communication with others of its kind. There are two main methods of ensuring that the neighbours know who is around, while at the same time possibly disclosing intimate details about sex and breeding condition: by shouting loudly about it, and by leaving smelly calling cards. The latter method is probably employed by all nocturnal prosimians and has the advantage of staying-power. It is also less risky, being a private communications channel decipherable only by the noses of other members of the same species and therefore less likely to attract the unwelcome attentions of predators than when sitting around in an exposed spot and calling loudly. If applied in a particular way a scent may be remarkably enduring, as in the Cheirogaleus dwarf lemurs which use their extruded anus as an applicator to smear a line of faeces along a branch, leaving an odorous 'squatter's rights' proclamation capable of sticking around for many weeks or even months. Bushbabies also leave scent messages, but use a multiple applicator in the form of the hands and feet. These are meticulously daubed with urine before the animal moves off, leaving at every step a gradually declining trail of smelly footprints which no doubt reveal a mine of information to the recipient.

Like most (perhaps all) nocturnal prosimians, bushbabies also use sound - up to eight different calls have been noted in the lesser bushbaby - to communicate with their neighbours; the calling of these diminutive animals can be one of the most characteristic sounds of the African bush after dark. Bushbaby society generally consists of several overlapping female home ranges which are more or less completely covered by the much larger home range of a single male. He makes it his frequent business to visit each female's range to check out her breeding condition, to be certain of being in the right place at the right time to mate with her if she is in oestrus. Although he may not be the only male in the vicinity, he is the largest and toughest, so probably secures sole access to the females. Rather more lightweight males manage to squeeze in a living between the ranges of the females and the central 'boss' male, while other males who have just reached adulthood lead the itinerant life of the drifter, passing at will through the ranges of both females and central 'boss' males. The presence of even smaller immature males may also be conceded within the range of the central male, presumably because they do not currently present any kind of sexual threat. So although bushbabies always seem to be alone when located in the forest at night, they do lead busy lives which include regular social contact with one another.

Horsefield's tarsier probably has similar arrangements in the forests of Borneo. Adult males seem to have larger home ranges than females, from around 8.75 to 11.25 ha (21.6 to 27.8 acres) for males and 4.5-9.5 ha (11-23.5 acres) for females. Unlike the galagines, whose females often sleep together during the day in a communal nest, both sexes of Horsefield's tarsier sleep alone. In fact all the evidence suggests that this species is perhaps the most solitary of all primates, regularly communicating with others of its kind only through calling and scent marking. However, there is a broad repertoire of calls, some rather insect-like, which serve to keep close neighbours aware of each other's presence. This may escalate to the point where at certain times veritable concerts may be struck up by several animals (up to five) perched in close vicinity. These concerts may be remarkably enduring, and in one case lasted for four hours. This concert style of calling seems to be quite different from the alternate duets found in the spectral tarsier and in unrelated animals such as the weasel lemur. The precise function of these tarsier concerts is unknown, but they may be connected with territorial boundary disputes. The exact nature of male-female relationships in Horsefield's tarsier is currently unknown; but it seems probable that, unlike in the galagines, two or more males may regularly visit the range of a single female from their own ranges, which only overlap with hers to a marginal extent. Scent marking may not be important and vocalizations are probably the main form of communication.